How trees affect garden & farm crops in drought

When assessing all the ways that extreme drought affects crops, a subtle factor to consider is the impact of tree roots on the water supply available to nearby plants. While gardeners and farmers tend to think of trees primarily in terms of shade (as competitors for sun), under these extreme drought conditions trees can become serious competitors for water, too. We’re seeing signs of this on our farm, and suspect many farmers & home gardeners are too without even realizing it.

The effect of trees on our crops is something we’ve thought loosely about in the past, as most of our growing areas are near tree lines; indeed we’ve actively worked to clear trees and brush back from several areas to earn more sun and growing space. In a few places, we’ve left large, beautiful trees that we’re sure are problematic but are too nice to remove. However, we’ve had a hard time truly judging the effect of trees on crops until this year.

Trees can pump thousands to tens of thousands of gallons of water every day through their roots. This is why heavy rains often cause worse floods in winter; because leaf-off trees don’t pump nearly as much water from the soil, causing that much more runoff into streams. In a nasty drought, the opposite is true. Most trees on our farm are highly stressed right now, with leaves turning yellow and blowing off; thus their roots are desperately seeking any water they can find and tree roots can extend well beyond the extent of the tree’s crown. Thus, once you think about it, the effect of nearby trees on crops, with or without irrigation, could potentially be huge.

An impressive example of this effect comes from a Kansas State pecan researcher, who recently wrote about the effect his pecan trees were having on nearby soybeans. Check out the amazing photograph and convincing explanation before reading further here.

We can demonstrate numerous examples of this effect on our farm, whose fields border individual trees and solid tree lines in multiple directions. First, consider our planting of fall pole beans, seen below in a view looking north.

Although the tree line is 20-30 feet west, there is a dramatic dropoff in plant health and growth toward the trees. Some of this may be due to water competition from the fescue aisle immediately west, as well, but the trees are a far larger influence. It’s not a sun effect, as the early evening western shade is mitigated by extra-early morning sun, and under these hot conditions some early shade is probably beneficial to the beans. We’re irrigating these beans, as you can see from the lush growth to the east, but with every bed west it’s clear the extended tree roots are just sucking all the water from the ground. In effect we’re trying to irrigate a whole stretch of forest edge with a few drip lines! Beans can’t be expected to out-compete trees for water. We saw a similar pattern with other crops just north but in the same line (including cucumbers and sweet corn) in which the beds closest to the trees performed noticeably worse. Again, there may be a partial fescue effect as well, but we’re pretty sure the trees are a big factor.

Our next example is a block of okra near the house. The above left view looks due south at a very severe decline in okra plant size and health toward the east. The above right view looks due north at the culprit: a large and beautiful maple tree that we’re perfectly aware is problematic for crops at times, but is too nice a tree to get rid of. What the photos don’t quite capture is just how well the pathetic part of the okra bed mirrors the rough root zone of the maple. It’s north of the okra, and not causing any shade problems, but that tree is sucking up every ounce of water it can find, and okra on drip lines is no match for a mature maple tree.

Even in future non-drought years, it’s conceivable this could become a worse problem. Tree roots don’t necessarily grow evenly in all directions; they trend in beneficial directions. For example, trees near roads will send fewer roots toward the road because there’s nothing there for them. So in an extreme drought year like this, are we teaching our trees to extend their root zones further into our growing areas so that they steal more water (& nutrients) in future years?

Finally, this situation may help explain another pattern we’ve been musing on: the prevalence of home gardeners we know commenting on poor results (such as here and here), while most professional vegetable farmers we know are having bumper years. Most home gardens, especially in urban areas, are quite close to multiple trees. Even if the gardener chose a sunny spot, trees to the north, east, or west could be sucking up all their irrigation attempts and making the plants perform far worse than in a farm setting where, in general, there’s less tree competition for water in larger open fields. Even if a gardener’s plants don’t look specifically drought-stressed or shriveled, the lack of sufficient water may still be enough to encourage other pests, diseases, fruiting troubles, and so on without any direct evidence pointing to the trees.

Something to think about, and one of many subtly, poorly-understood, and long-term effects of this exceptional drought we’re in.

4 thoughts on “How trees affect garden & farm crops in drought

  1. We’ve talked before about the heat island effect delaying frost in urban areas. Do you think it could also cause those of us in urban areas to experience even higher temperatures and less cooling at night in an extreme heat situation like this summer compared to farms, which are generally in rural areas?

  2. Annette,

    I’m not sure whether landscape fabric truly blocks all roots; if it does, I’d be afraid of the effect on your own plants as many vegetables can put down roots much deeper & longer than 16″.

    Regardless, landscape fabric doesn’t block moisture movement. The tree-root pumping creates a negative water pressure that could still draw soil moisture out of your beds, especially as water wants to move down anyway. Think of it like the draw-down of groundwater around a well; similar effect. So, if you have trees in the vicinity, I’d still expect that tree roots underneath or even in the neighborhood of your beds could still be drawing water out of them. Can’t easily prove it, and high organic matter in your beds could buffer the effect due to high water-holding capacity, but it doesn’t seem to me that raised beds would be inherently immune to the effect of tree-root water draw.

    I definitely agree that urban heat island is a factor as well.

  3. Good to know. We have only a small columnar peach in our yard, but there is a nearby magnolia tree and huge oaks and maples in neighbours’ yards. When we moved part of one of our beds, we did find roots from the magnolia making their way into the growing area of the bed.